Posted: Tue Mar 19, 2013 12:24 am Post subject:
Power supply problem + - 12VSubject description: where there should be +12v there is almost nothing

Hi, I've built the wall wart power supply, and the negative rail is working, but the positive rail only outputs 0.8V.

I have used 3k load resistors, 1N7007 diodes, 3300uf 25v capacitors, 35v tantalums, a "7812CZ" and a "7912CT".
The AC adapter is 500mA.

I also adapted the circuit board design to be single side, but I can't see any problems with it. The capacitors are quite large and axial type so I've stacked them on top of each other, which is why there are only pads for one pair.

It's most likely the known problem with some makes of regulators. Search the forum for 'power and latch' and you'll find previous experience. Unplugging the power supply and plugging back in should sort it - you may need to do that more than once.

If they look "old" replace them. If they've been sitting in a parts drawer for 15 years, and/or have never been used, that makes them even more sus.
NOS electrolytic caps should be avoided like the plague. _________________What makes a space ours, is what we put there, and what we do there.

My best bet is that you have C3 or C4 round the wrong way
Disconnect the output, remove CR3 and CR4 and check again.
Still not right? Then remove C1, C2 and C3, checking the output voltage after each one.
You should now just have C4, C5 and R2 around the regulator.
Pin 1 should be at around 16V +/-2v and the pin 3 should be 12V.
If not its either a dead 7812 or a PCB fault. Check again for dead shorts or cracked tracks._________________Stephen
www.Rainsbury.Net

Thanks very much, I tried it but no luck, there was just a small voltage at pin 1.. I can't see anything wrong with the circuit board, so I will keep trying with voltage regulators. I also ordered a couple of the big capacitors just in case.

In any case do you think you could explain a bit the process of removing those parts, like why you would remove them in that order and what you would be looking for? At this stage I am mostly just following the instructions and it would help me understand a bit more about the circuit

The process comes from years of working with them
These regulators only actually only need two capacitors to work, in this case C1 and C5, an that is how they would normally be used however in most cases they would be permanently connected to a circuit with a known load current.

My normal method is to remove everything that's not essential bit by bit until it starts working or there is nothing left.

In this case the diodes are there to protect against connection problems, so if the circuit isn't connected you don't need them. If they had been the wrong way round they could have caused the problem you saw.

The next thing was the the really big capacitors are not needed if you are not drawing a large current, so again if the circuit is not connected to a load you don't need them.

Electrolytic capacitors degrade as they get older and can either create a short circuit or present some random value so remove them to eliminate the possibility.

If you remove the regulator then only CR1 and C4 should be connected. This would be a simple half wave rectifier, and teh DC voltage across C4 should be (wall-wart voltage x 1.4) -0.7V. If its not then one of them is dead so you need to fix that first.

Connecting the regulator should not cause this voltage to drop much, if it does the regulator is either dead or the output is shorted.

The Regulator output should be 12V, if not and the input voltage is correct then it must be a dead regulator, or a PCB fault.
If all is OK then keep adding bits back until it stops working, at which point you have found the problem.

This philosophy works for most circuits, but relies on you understanding
it and being able to work out what can be removed, and that just comes with time._________________Stephen
www.Rainsbury.Net

LOL there is 8V difference between each side of CR1
Thanks so much for your help, I think i will rebuild it bit by bit now to make sure there are no other bad parts.
With the old-looking electrolytic capacitors, should I connect the circuit to a load after putting them in to check they are working?

Ah ok thanks a lot, so I took out everything but CR1 and C4, and there was 20V there, but after I put the regulator back, the input voltage dropped to 8V and the output to 6V.

I would say then that the problem is the regulator or the PCB around the regulator. Without the regulator, what voltage do you have at the output? Without the regulator could you check if there is continuity between any of the three pads of the regulator?_________________electronic-sea.net

I put the regulator back, the input voltage dropped to 8V and the output to 6V.

Quote:

can you measure the voltage drop across CR1? It should be around 0.7V

mmm... Actually not. Notice that this power supply is a half-wave rectifier but that the same secondary is feeding both rails. Therefore, if pico.farad is measuring DC, from ground to the anode of the rectifier, the voltage would be 0. And across the rectifier, he is actually measuring the 8 volts measured previously.

pico.farad, are u testing with no-load? what is the voltage rating of the wall-wart? Does it have any data-sheet or specifications?_________________electronic-sea.net

yes you are right I Should have said was the anode cannot be more than 0.7v more positive than the cathode and I forgot that this would be difficult to measure with a meter. (I use a CRO and forget sometimes)

Any PCB fault is going to be mechanical damage, so either damaged track or short, but there are so many possibilities there it would really need a photo.

I agree that its probably a dead but 7812 are normally pretty bomb proof and I have never managed to blow one up, although I have picked up several stories recently them dieing and put it down to cheap copies getting onto the market._________________Stephen
www.Rainsbury.Net

yes you are right I Should have said was the anode cannot be more than 0.7v more positive than the cathode and I forgot that this would be difficult to measure with a meter. (I use a CRO and forget sometimes)

You were right, measuring the peak it would be correct. I guessed that pico.farad was measuring DC.
Therefore, the voltage peak across the secondary would be something like 9Vpeak.
I would say that there is something shorting to ground and that the transformer drops its voltage due the voltage regulation.

I would check if there is continuity between the output and the ground and between the input and the output. Also I'd recheck if the protection diodes are placed correctly._________________electronic-sea.net

You guys are right, this whole time I was measuring DC with a multimeter...
Also it was with a 12v adapter and no load. I have been testing around the circuit board for shorts/opens but I can't find anything. I feel it's quite likely the regulator is a cheap copy though, I'll try to find a legit one.

I finally found another regulator, and I am testing with only CR1 and C4 connected, plus the regulator. Measuring DC with no load there was 9v coming in and 7v going out, which i think is similar to last time. Then I added the load resistor R2 and there was 7v going in and 5v going out.

I think I am still lost.. does this mean there is a short to ground somewhere?

I finally found another regulator, and I am testing with only CR1 and C4 connected, plus the regulator. Measuring DC with no load there was 9v coming in and 7v going out, which i think is similar to last time. Then I added the load resistor R2 and there was 7v going in and 5v going out.

I think I am still lost.. does this mean there is a short to ground somewhere?

Try to connect C1.
Could you check the value of R2?
What is the value of C1?_________________electronic-sea.net

keep in mind that for a +/-12V using standard regulators is recommended a 15VAC transformer instead of the 12VAC you are using. You can read more about this here.

And keep in mind that for the rectifier configuration of this power supply, according one measurement I did, the maximum power output is about 125mA.

You can check it if you have a multimeter reading AC RMS and a few power resistors of different values. For example with two resistors of 100Ω you can check the current flowing in the secondary of the wall wart for a load of 120mA and for a load of 240mA. They should have capacity greater than 1.5W for a simple measurement. I will check it later, no time now. I would be very interested in the results of your measurements.

If you don't have power resistors, you can connect three or four modules to the power supply and read the AC RMS current in the wall wart's secondary._________________electronic-sea.net

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